1. Field of the Invention
This invention relates to a switch device of the type where its operating knob is swung to trip the movable contact piece.
2. Related Art
The arrangement of a conventional switch device of this type is as shown in FIGS. 6 through 8.
As shown in FIG. 6, stationary contacts 2a and 2b, and an L-shaped common contact 3 are provided in a casing 1. A movable contact piece 4 is placed on the front end of the common contact 3, in such a manner that it is swingable about a center of swing 4a located on the common contact 3. In addition, an operating knob 5 is mounted on the casing 1 in such a manner that it is swingable about a shaft 5a. The operating knob 5 has a pusher 6 which is adapted to push the surface of the movable contact piece 4. The movable contact 4 is to be pushed by its upper surface 4b. The pusher 6 is urged towards the movable contact piece 4 by a compression coil spring 6a.
In the switch device thus constructed, when the operating knob 5 is at the neutral position, the movable contact piece 4 is held downward towards the right as indicated by the solid line in FIG. 6; that is, it is in contact with the stationary contact 2a. Under this condition, the operating knob 5 is swung in the direction of the arrow A, the pusher 6 is swung in the same direction, and at the same time the end portion of the pusher 6 is slid on the upper surface 4 of the movable contact piece 4b.
As shown in FIGS. 7 and 8, the point of intersection of the upper surface 4b and the perpendicular .alpha. drown from the center of swing 4a of the movable contact piece 4 to the upper surface 4b is the dead point 7. Hence, as indicated by the one-dot chain line in those figures, the movable contact piece 4 is not allowed to swing until the depression point (contact point) of the pusher against the upper surface 4b of the movable contact piece reaches the dead point 7; that is, the movable contact piece 4 is swung in the direction of the arrow C when the depression point reaches the dead point 7. The movable contact piece 4 is swung with the movement of the pusher 6, and the swinging operation is ended when the left end portion of the movable contact piece 4 is brought into contact with the left stationary contact 2b as indicated by the two-dot chain line in FIG. 6. At the end of the swinging operation of the movable contact piece, the depression point of the pusher 6 against the upper surface 4b is indicated at 8, and the perpendicular dropped from the center of swing 4 is indicated at .alpha.'.
The above-described switch device suffers from the difficulty that, when the movable contact piece 4 is operated into and out of engagement with the stationary contact 2a or 2b, arcs may be produced between them to consume the stationary contact 2a or 2b. This is significant when the switch device required to interruption of a fault current. Thus, in order to decrease the consumption of the contacts, it is essential to shorten the period of time in which the arcs occur; i.e., to increase the opening speed of the switching contacts and this reduces consumption of the contacts.
In the above-described conventional switch device, the center of swing 4a is provided on the lower surface of the movable contact piece 4; that is, the distance between the center of swing 4a and the depression point of the pusher 6 against the movable contact piece 4 corresponds to the thickness of the movable contact 4. Hence, it is necessary for the pusher 6 to move through a stroke L for the period of time from the time instant that the movable contact piece 4 starts swinging until it ends it. The stroke L is approximately represented by the following expression (1): EQU L.apprxeq.t.times.tan .theta. (1)
where t is the distance between the center of swing 4a of the movable contact piece 4 and the depression point of the pusher 6 against the movable contact piece 4 (i.e., the thickness of the movable contact piece 4); and .theta. is the angle of swing of the movable contact piece 4).
As is apparent from the above description, the angle of swing of the pusher 6, and accordingly the angle of operation of the operating knob 5 is large, and the contact switching speed S of the operating knob 5 is slow. The contact switching speed S can be calculated according to the following equation (2): EQU S=l/{(V/.pi.d).times.(.phi./180)} (2)
where V is the operating speed of the operating knob 5, d is the distance between the shaft 5a of the operating knob 5 and the end of the pusher 6; l is the distance between the movable contact piece 4 and the stationary contact 2b; and .phi. is the angle of swing through which the pusher 6 swings for the period of time from the instant that the movable contact piece 4 starts swinging until it ends.
For instance in the case where V=90 mm/s, d=20 mm, and 1=1.3 mm, ##EQU1##
In the above-described switch device, the stroke L of the pusher 6 is long, and the angle of swing is about two (2) degrees. Therefore, when .phi.=2 is substituted in the equation (3), the contact switching speed S is as follows: ##EQU2##